In many industrial and mining operations, the intermediate or the finished product is a granular material which often consists of a mixture having components of varying density and porosity. In many situations, it is required that the valuable components from the mixture be first separated before its end use. In many such instances, it is possible to separate the components and recover the valuables by physical processes such as gravity separation, screening, etc. However, when the density difference between the various components of the mixture or their particle size or properties other than those described herein is not significant, then the process of separating it into its components not only becomes difficult, but is also ineffective. However, the difference in the density of one component over the other can be significantly increased provided the porosities of the individual components is different. Separation of coal from its organic impurities, commonly known as ash, is such an example.
Coal is a heterogeneous mixture of organic (macerals) and inorganic (mineral) matters. The distribution of its components and its chemical compositions varies widely with the geographic location of the coal and coal type. The inorganic content of the raw (freshly-mined) coal varies from a few percentage points to as high as 40%, and it consists largely of shales and clays, calcium and magnesium carbonates and sulfides of iron known as pyrites. The bulk of this inorganic impurities in raw coal falls into the category of "extraneous" ash. The extraneous ash includes the sedimentary material laid down as "dirt bands" on top of the plant remains which, with time, was converted to coal, mineral deposits (usually carbonates) from percolating solutions late in the coal-forming process, and the roof and floor materials that are usually mined along with the coals, especially in the continuous mining coperations. These impurities are not bound tightly to the coal and, therefore, a significant portion of it can be liberated from the coal by crushing and grinding operations.
Two properties of coal macerals that differ from those of the mineral impurities form the basis of virtually all the physical coal cleaning processes: specific gravity and the surface characteristics. Based on the existing knowledge of coal and its impurities, it can be stated that the organic portion of the coal is less dense and more hydrophobic than its inorganic impurities. Table I illustrates the point on the density differences:
TABLE I ______________________________________ Specific Gravities of Coal and Its Mineral Impurities Component SP. Gr. ______________________________________ Coal 1.3 to 1.7 Shales and Clays 2.0 to 2.7 Calcium and 2.0 to 2.5 Magnesium Carbonates Pyrites 4.5 to 5.0 ______________________________________
The major commercial cleaning processes for coal crushed into a particle size range of 0.01" to 2" are based on density differences, whereas methods based on surface characteristics such as froth flotation, selective flocculation, etc., are used for coal particles having a size range between 0.001" to 0.01".
Despite the density and hydrophobicity differences between the coal and its inorganic impurities, the extent of separation achieved by existing commerical techniques cannot be termed exceptionally successful. Table II shows the extent of separation obtained on coals of three different ranks based on the present "float and sink" method.
TABLE II ______________________________________ Gravity Separation of Coal Gravity Fraction 1.3 .times. 1.7 1.7 .times. 2.1 2.1 (sink) ______________________________________ Coal A: Rank: High Volatile A Total Composite Ash: 13% weight % coal 67 14.5 18.5 % ash 9.2 17.2 33.0 (on material retained in each gravity fraction) Coal B: Rank: High Volatile C Total Composite Ash: 9.5% weight % coal 91.0 6.5 2.5 % ash 6.0 36.9 74.2 (on material retained in each gravity fraction) Coal C: Rank: Sub-bituminous C Total Composite Ash: 12.0% weight % coal 95.0 3.0 2.0 % ash 6.2 31.0 74.3 (on material retained in each gravity fraction) ______________________________________
It can be noted from Table II that the fraction of coal retained in the gravity fraction of 1.3.times.1.7 has lower ash content than the parent coal, whereas the ash in the higher gravity fractions are higher. Thus, the coal separated in the 1.3.times.1.7 gravity fraction is richer in organic content and leaner in impurities. In other words, the coal has been beneficiated. It is to be further noted that even though the inorganic impurities has been reduced in the beneficiated fraction, the reduction in the ash content for Coal C has been about 50%, whereas in the case of Coal A and Coal B, which are of more commercial importance than Coal C, it is even less than 35%. The extent of beneficiation by the float and sink method cannot be, therefore, termed exceptional.
The organic fraction of coal is known to be highly microporous, whereas the mineral impurities it contains are primarily non-porous. For the purpose of this disclosure, a microporous substance is defined as that fraction of the void of a specific volume of the material that is contained in pores ranging in equivalent cylindrical diameters between 3 Angstroms to 100,000 Angstroms. The organic fraction of a coal, because of its porosity, adsorbs significant quantities of such fluids as air, other gases, vapors and liquids. It is due to this microporosity that coal can contain up to 2200 cubic feet of methane per ton of its weight. It is because of this inherent microporosity that materials of high internal surface area, such as activated carbon, can be produced from coal.
One of the measures of microporosity of coal is its internal surface area, which can be measured by adsorbing carbon dioxide and by employing the Dubinin-Polanyi equation. Other measures of microporosity are the differences between the X-ray, helium and mercury densities. The surface areas of some coals, as measured by adsorbing carbon dioxide and employing the Dubinin-Polanyi equation are compared in Table III.
TABLE III ______________________________________ Surface Areas of Various Coals Determined from Carbon Dioxide Adsorption Coal Type Surface Area (m.sup.2 /g) ______________________________________ Anthracite A 238 Anthracite B 274 Medium Volatile 133 Bituminous A High Volatile 144 Bituminous C ______________________________________
As can be seen from Table III, the surface area of coal is generally in the hundreds of m.sup.2 /g. The surface areas of the inorganic impurities, on the other hand, hardly approach 10 m.sup.2 /g.